I knew very little about pennyroyal, except that it seems to keep the ant population down in a large stone planter I have. Six feet long by 2 feet wide & 3 feet deep to ground level, it has been a hotbed of ant breeding for 15 years or so. Anything that grew there was by courtesy of millions of ants. I put two creeping pennyroyals in...and for the last two years there have been considerably fewer massive colonies of ants. Some have moved underground and over to the veggie patch, but that's beside the point. Ants don't like pennyroyal, so that's my starting point.

Certainly the essential oil used topically or the fresh leaves crushed and rubbed onto the skin will ward off mosquitoes and fleas (see section IX). Philbrick & Gregg, in their ancient & treasured Companion Plants agree. They also state that the American pennyroyal yields a commercial oil which can repel gnats & mosquitoes. Soak a dog collar in an infusion of pennyroyal or add a strong decoction into the floor washing water are well regarded folklore remedies by Adele Dawson. Richard Mabey claims pennyroyal is also good with bites of all kinds, repelling ticks as well as the above. Tierra suggests using citronella oil with pennyroyal oil for external application against mosquitoes.

Topically, it is a refrigerant, antiseptic, insect repellent, and thus good for skin eruptions, itching, formication [the sensation of small insects crawling all over the skin] & gout [presumably for its cooling property applied to the affected, "burning" joint in an acute attack]. Parenthetically, it is only the British Herbal Pharmacopoeia that includes gout in the pennyroyal portfolio.

I. History of the name

from Malcolm Stuart's Encyclopedia of Herbs & Herbalism
Pennyroyal was held in very high repute for many centuries throughout Europe & was the most popular member of the mint family. Pliny is regarded as the originator of its name "pulegium" ...derived from "pulex" meaning flea...since both the fresh herb & the smoke from the burning leaves (smudging) were used to eradicate the insects. Linnaeus retained the association with fleas when he gave the plant its botanical name. Prior to that scientific classification, the unusual aroma led some to consider it a thyme.

"Puliol" was an old French name for thyme, & this plant was designated the royal thyme or "puliol royale" which was corrupted into pennyroyal. In modern French, the herb is called "la menthe Pouliot".

Herb books written in the US tend to list American Pennyroyal (Hedeoma pulegioides) first , and medical herbals written in the UK & Europe prefer Pennyroyal (Mentha pulegium). All agree the <other> pennyroyal has the same or similar properties. Where the real differences lie are in the appearance & life cycle of the two herbs.

II. Description of pennyroyal

Pennyroyal (Mentha pulegium) is an aromatic Perennial and is common wild or garden plant in the UK, France & Germany; found in wet grounds around the Med & in Western Asia. Mrs. Grieve says the more common, at least in the UK, is the creeping or <decumbens> pennyroyal. With weak, prostrate stems, though quadrangular -- as all mints have square stems -- it roots easily where ever the leaf nodes touch the ground. H 10-15cm or 4-6in. S indefinite. <Erecta> the upright or sub-erect has stouter stems, & obviously there's no rooting at leaf nodes. It is less common in the UK but better for cultivation according to Mrs. G. H 20-30cm or 8-12in. S indefinite. A planting will last 4-5 years, though Mrs. G says frost may kill it, & a new planting should be made each year. Deni Brown lists pennyroyal as fully hardy [minimum -5C or 5F). Ethne Clarke's Herb Garden Design shows pennyroyal appropriate for Zones 5-9.

The leaves of Pennyroyal are generally small, ovate, slightly serrate, slightly hairy, and opposite. For the record, the leaf of the non-creeping pennyroyal can be up to 3cm or 1.5in long and may be entire rather than slightly toothed. The color depends on the variety and whether wild or cultivar. Greyish-green to light green. The IMPRESSION of the appearance of the leaves is similar to that of wild oregano (Origanum vulgaris), marjorams (O. majorana, O. onites) & thymes...that is... tiny & crowded together on thin stems but with more rounded leaves. Not surprisingly Mrs. G described pennyroyal as <the smallest of the mints & very different in habit>.

The small flowers are produced in distinctive, dense whorls (similar to corn or fieldmint & gingermint in bloom. ) The tight, axillary clusters appear in July-August with colors ranging from reddish -purple to lilac. There are few flowering stems on the prostate form; they lie on top of what appears to be "a dense green turf". Seed is light brown, very small & oval.

To harvest: for drying, the stems should be gathered just before flowering in July. Pungently aromatic, it can be added to potpourris & insect sachets. The dried herb can also be made into infusions, liquid extract, tinctures for medicinal uses. (see section X)

III. Description of American pennyroyal

According to Deni Brown, there are 39 species of annuals & perennials in the NAmerican genus, Hedeoma. They have no great merit as garden plants, but are often seen in herb gardens. Its neat habit & aromatic foliage makes it especially suitable for containers & planting near seats & entrances, or between paving stones.

American pennyroyal is an Annual, found in dry fields & open woods from the East coast to Minnesota/Nebraska. It is bushy plant with erect, square stems. H 10-40cm (4-16in) S 7-24 cm (3-10in) it bears small, opposite, thin ovate leaves sparingly toothed. Axillary clusters of small, tubular lavender or purplish flowers appear from June-October. The whole plant has a pleasant, aromatic, mint-like smell. The name Hedeoma comes from the Greek <hedys> for sweet and <osme> for scent. It has also been described as having an acrid taste and aroma; none-the-less it is used as the basic flavoring herb of North Carolina black pudding... hence the local name of Pudding Grass.

A culinary aside: In the north of England, Pennyroyal (Mentha pulegium) is also used in black pudding, and in Spain it is added to sausages.

To harvest: plants should be cut when in flower for drying. The fresh herb can be gathered and used almost as a "strewing herb" for deterring fleas.

IV. Therapeutics of Mentha pulegium

For the basic framework, I am using the information from the British Herbal Pharmacopoeia, 1983 supplemented by Potter's, Culpepper's Colour, David Hoffmann, Deni Brown, & Simon Mills.

In small doses & as an infusion, pennyroyal is used for colds (as it promotes sweating), With its richly aromatic volatile oil, pennyroyal will ease indigestion, wind, nausea, colic, dyspepsia, and painful menstruation. It is considered a warming & stimulant herb by Culpepper, while Adele Dawson also suggests its use in cases of stomach spasm & hysteria. Hoffmann explains that the volatile oil will relax spasmodic pain & ease anxiety. NB: This should NOT be construed as ingesting the isolated, essential oil which could be fatal. (see section IX) The volatile oil is a constituent of the plant & will be released in the preparation of the infusion.

Pennyroyal is given to children with stomach & bowel upsets & also to ease feverish symptoms in measles & whooping cough. Taken by infusion according to Culpepper.

BHP suggested dose: for an infusion: 1- 4gm of dried herb in 1C ** boiling water; steeped for 10-15 minutes. 3 times a day.
Or 1-4ml of liquid extract (1:1 in 45% alcohol). 3 times a day.
Other herbals consulted tend to suggest smaller doses: up to 2 or 3gm dried herb; up to 2ml tincture...though Potter's range is from 0.5ml - 5ml of the liquid extract.
**NB: the general proportions for infusions are: 30gm dried herb or 75gm fresh herb to 500ml boiling water. So "one cup" is the proverbial length of a piece of string.

Pennyroyal is available on the General Sales List in the UK. [In itself, that is an indication of its considered safety.]

V. Therapeutics of Hedeoma Pulegioides

Properties & uses: carminative, diaphoretic, emmenagogue, sedative, expectorant. The Amerindian tradition shows use of pennyroyal for headaches, feverish colds, & menstrual cramps & pain. It was also used as a digestive herbal tea. It was listed in the US Pharmacopoeia (1831-1916).

It is still used internally for colds, whooping cough(the expressed juice can be made into a lozenge/sucking candy). In childbirth, the PLANT is used.
NB: the essential oil taken internally could be fatal. (see section IX). It should be used by qualified practitioners only.

Pulegone is described as a toxic compound, "notorious for causing abortions". It is present in both Mentha pulegium & Hedeoma pulegioides.

VII. Additional medical interpretations

Tierra in Planetary Herbology collectively describes Hedeoma pulegioides & Mentha pulegium ...in much the same way as mentioned above. He does add the following: The Energetics are spicy, bitter, warm . The Meridians/organs affected are liver & lungs.

In David Bellamy's & Andrea Pfister's World Medicine they have a large section called The Families of Healing Plants. Mentha pulegium is listed with two sources of information: The 1907 British Pharmaceutical Codex and Book I of Avicenna's Canon. The BPC states that Oil of Pennyroyal (Ol. Pulegii) is given as an emmenagogue. During excretion, it mildly irritates the kidneys & bladder, and reflexly excites uterine contractions. Avicenna lists the herb as Mint (Podina in Urdu). The leaves are the part used. The herb's Temperament is described as Hot & Dry in the 2nd Degree.

VIII. How deadly is deadly ... and which pennyroyal are we talking about?

Simon Mills in Out of the Earth issued the strongest warning that I found in my trawl of herbals. There are a number of herbs which should be avoided altogether because they can damage the fetus or provoke a miscarriage. In many popular herb books the term emmenagogue is found, widely but erroneously, having come to refer to a gynecological remedy. In fact, the effect of an emmenagogue is to bring on a delayed menstruation: it takes little imagination to realize that the most common reason for a delayed menstruation is pregnancy and that emmenagogues are thus abortifacients. Pennyroyal (Mentha pulegium) is among 21 herbs Simon Mills lists. This information was part of a short section of herbs in pregnancy..those quite safe, and those not so.

In his first book, The Dictionary of Modern Herbalism written 6 years earlier, Mills quite carefully does NOT list emmenagogue among the actions. He does include uterine stimulant with this caution: <pennyroyal should not be used in pregnancy or when any delayed menstruation might denote pregnancy; it is as likely to damage the fetus as procure the abortion.>

The second most complete, cautionary listing was found in Earl Mindell's Herb Bible.
He is talking about American pennyroyal (Hedeoma pulegioides).
<Back in the days when abortion was illegal, this herb was used to induce abortion. In some cases, it resulted in hemorrhaging & serious complication for the mother. Therefore, it should never be used for this purpose. Today, pennyroyal is one of the herbs used by herbalists to facilitate labor & delivery. It should be used only under the supervision of a knowledgeable practitioner. If you do use this herb, do not exceed the recommended dose & do not take for more than a week at a time.>

Richard Mabey in The Compete New Herbal warns <...the oil taken internally can be highly toxic and there are a number of cases of the deaths of women who tried to procure abortions by taking the oil.>

Tierra in Planetary Herbology goes further in his explanation. <To take the oil internally to terminate an unwanted pregnancy is very dangerous, and in a few cases has resulted in death. All essential oils are life-threatening if taken internally. There is a possibility of fetal damage from the use of pennyroyal (Hedeoma pulegioides or Mentha pulegium) to induce abortion, but this may be true only of the undiluted oil and not the infusion.>

Malcolm Stuart raises an important, tangential danger. <Although long considered an abortifacient, it has been found that this effect is usually only possible with a dose of the oil which is highly toxic and leads to irreversible kidney damage.>
He then goes on to state:
<The plant oil can therefore be used as a flavoring agent, but only when the concentration of pulegone does not exceed 20mg parts per 1kg of the final product being flavored.>

IX. PS on pests

And just to round things out, he adds that the plant may cause contact dermatitis which is certainly worth noting before rubbing crushed, fresh leaves on your skin to avoid mosquito bites. He adds that the pennyroyal leaves are also good for insect bites after-the-fact. They act as a rubefacient...that is drawing more blood to the area which improves its cleansing action on the affected tissue.

X. Odds and sods

While some herbalists maintain that a fresh herb/plant is medically more efficacious, I have not seen any preferences specified for either Pennyroyal or American Pennyroyal. Most herbals referred to the dried herb...so by omission one can assume dried is the preferred state. Why? I don't know.

Forms of internal dosage:
The British Herbal Pharmacopoeia, Mrs. Grieve, & Potter's call for a liquid extract to be taken. Hoffmann & de Baiiracli Levy use infusions. Lust calls for a tincture to be used.

As a reminder, the differences are:
Tincture: solution of substances (both active & inactive therapeutically) extracted from medicinal plants by the maceration or percolation of the plant with alcohol or alcohol-water solutions.
Liquid extract: product obtained by treating plant material with a solvent or mixture of solvents designed to extract the desired constituents.
Infusion: made by pouring a given volume of boiling or just boiled water over a given quantity weight of herb and letting it steep/infuse for a given time.
Always cover to keep the volatile oils in the infusion...otherwise they'll escape...evaporating into the air.